Out CNN anchor Anderson Cooper was surprised to learn that his mother once had a same-sex dalliance.

The news host is the son of famous heiress, actress and author Gloria Vanderbilt. The pair have filmed a documentary and co-written a book about their relationship… but Vanderbilt dropped a bombshell on her son during a press junket.

In an interview with PEOPLE Magazine, 92-year-old Vanderbilt was speaking about her son’s sexuality when she revealed that she had her own same-sex romance as a teen.

She said: “I, myself, when I went to [Miss Porter’s School in] Farmington, [Connecticut] I went through a brief so-called lesbian relationship with a girl in school. We all did – it was very prevalent.”

Cooper reacted: “What?! Hello! This is news to me… you didn’t mention this in the book, mum!”

She joked: “Well, we were in such a rush to get it out, I didn’t have time!”

“Cynthia, her name was, and she came once to visit my aunt in New York on holiday.

“We had this sort of lesbian relationship and it felt so great.

“It felt so great and yet I thought, ‘there’s something about this’.. I thought, ‘No, this is something that’s not really what I want.’ It was very brief.”

“I think almost everybody goes through at one point, whether they turn out to be for ever and ever.

“Of course, the thing is, now we realize there’s no difference. Love is love.”

Indeed, Ms Vanderbilt’s lesbian fling would once have dominated tabloid headlines across the world.

Cooper himself became the news when he came out as gay in 2012, ending a long-standing silence on the subject.

He explained at the time that he had stayed silent due to his desire to maintain political neutrality, as well as to keep safe when reporting from countries where gays are persecuted.

But the news anchor said he had come to consider whether “the unintended outcomes of maintaining my privacy outweigh personal and professional principle”.

Cooper said he was “distressed” that his silence could be construed as him “trying to hide something – something that makes me uncomfortable, ashamed or even afraid”.